fix: ignore unchanged bounty comment edits

[GHX5T-SOL]

Summary

• skip enqueueing another bounty response when an edited /bounty comment keeps the same amount
• keep real bounty amount edits working while allowing users to add context to the original comment without another bot response

Fixes #141

Validation

• TEST_DATABASE_URL=postgresql://postgres:postgres@localhost:5432/postgres mix test test/algora_web/controllers/webhooks/github_controller_test.exs:166 --trace - 1 test, 0 failures
• TEST_DATABASE_URL=postgresql://postgres:postgres@localhost:5432/postgres mix test test/algora_web/controllers/webhooks/github_controller_test.exs:137 --trace - 1 test, 0 failures
• mix format --check-formatted lib/algora_web/controllers/webhooks/github_controller.ex test/algora_web/controllers/webhooks/github_controller_test.exs
• git diff HEAD~1..HEAD --check
• git show HEAD | gitleaks detect --pipe --redact --no-banner - no leaks found

Note: I also attempted the full webhook controller test file locally with --max-cases 1; it failed before assertions with Oban/Ecto sandbox ownership errors from background Oban processes, so I validated the changed regression and existing amount-edit path independently.